1. Technical Field
This invention relates to power conversion. More specifically, this invention relates to the cancellation or reduction of a common mode noise waveform.
2. Discussion of the Related Art
In a simple AC power distribution system, there are three wires in a cord connecting, for example, a computer to a wall socket, which distributes the AC power. There is an active wire, a neutral wire, and a ground wire. Common mode noise is present on both the active and neutral wires and may be measured with respect to ground. The term “common” refers to the fact that identical noise appears on both the active and neutral wires. In some situations, common mode noise may be created by lightning, circuit breakers switching, poor grounding, or use of surge protectors that divert noise from the neutral wires. In high-frequency switching mode power supplies, common mode noise is created by the high frequency switching device within the power supply.
Common mode noise presents a problem because the common mode noise attempts to dissipate its energy from neutral to ground or from active to ground. In switching-mode power supplies, common-mode noise may be coupled through a high-frequency transformer or along paths that have stray or parasitic capacitance. Under certain conditions, especially if the common-mode noise consists of high frequency impulses, there is a probability that the noise will see the high frequency transformer as a coupling capacitor and pass through the transformer unobstructed. The power supply may also act like a high-frequency radio antenna, which may result in the power supply not meeting electromagnetic interference (EMI) standards. In addition, in small form power supplies, more stray capacitance paths may exist simply because the power supplies are smaller in physical size and more densely packaged when compared to other power supplies.
If common mode noise is transferred through a switching-mode power supply, a noise voltage appears between the ground and the voltage-supply pins of the device being powered. If the noise exceeds the maximum voltage specification of the device being powered, the energy from the common mode noise may pass through the logic hardware to ground, dissipating energy along the way. Reduced reliability, interference with data processing, and permanent damage may result. The magnitude of the common-mode noise does not need to be high to cause damage because electronic components in the device being powered may be able to withstand only a few volts or a few tens of milliamperes of current.
FIG. 1 illustrates an AC to DC power supply utilizing a noise cancellation circuit, as disclosed in U.S. Pat. No. 6,850,423. The AC to DC power supply 100 may include an AC input 116, a rectifier 118, a switching device 101, a transformer 110, a set of cancellation secondary windings 108 and 109, a regulating device 113, and a regulator 122. The power supplied is coupled to an output load 120. The transformer 110 may include a core 102, a primary winding 104 and a first secondary winding 106. The transformer 110 may include an inherent parasitic capacitance, e.g., C1, 111 representatively coupled between the primary winding 104 and the first secondary winding 106. In an embodiment of the invention, the AC to DC power supply 100 may also include a capacitor C2 112 which is coupled between the secondary winding 106 and one of the set of cancellation secondary windings 109.
Generally, the operation of the AC to DC power supply is as follows. The rectifier 118 may receive an AC input voltage from the AC input 116. The rectifier 118 may output a DC voltage. The switching device 101 may receive the DC input and produce a switched output. In embodiments of the invention, the AC to DC power supply may include one or more switching devices 101, depending on the configuration or design of the AC to DC power supply. For simplicity, the remainder of the application illustrates only a single switching device. The switching device 101 may also create a common mode noise waveform because of the high frequency operation of the switching device 101. The common mode noise waveform may be any shape waveform, e.g., a sqaurewave. The primary winding 104 of the transformer 110 may receive the switched output and the common mode noise waveform. The switched output may be transferred to the first secondary winding 106 and produce a transformed output. The transformed output may be input into a regulating device 113 which produces a regulated DC output. The regulated DC output, Vout, may be transferred to the load 120. A voltage regulator 122 may tap off the regulated DC output to verify that the regulated DC output is operating within a specified range. If the regulated DC output is not operating within the specified range, the voltage regulator 122 may transmit a correction signal to the regulating device 113 to modify the magnitude of the regulated DC output. The voltage regulator 122 may also receive a programming voltage or a programming current. The regulator 122 may verify that the regulated DC output is operating within an established ratio of regulated DC output to the programming voltage or the programming current. If the regulated DC output is not operating within the established ratio, the voltage regulator 122 may transmit a correction signal to the regulating device 113 to modify the magnitude of the regulated DC output.
The common mode noise waveform created by the high frequency switching device 101 may be capacitively coupled via parasitic capacitance 111 from the primary winding 104 to the first secondary winding 106. As discussed, the common mode noise waveform may cause the AC to DC power supply to act like a radio antenna and transmit common mode noise to the load 120. Thus, it is important to minimize or eliminate the common mode noise waveform. Although the parasitic capacitance is not embodied in a physical device, it acts as a real component of a transformer 110. The turns ratio of the primary winding 104 to the first secondary winding 106 may not determine the magnitude of the common mode noise waveform because the common mode noise waveform is capacitively coupled from the primary winding 104 to the first secondary winding 106. In other words, in embodiments of the invention, the magnitude of the common mode noise waveform on the primary winding 104 may be approximately the same value as the magnitude of the common mode noise waveform on the first secondary winding 106 because it may not be reduced by the turns ratio of the primary-to-secondary windings. Instead, the common mode noise waveform may be directly coupled to the primary winding 104 via the inherent parasitic capacitance 111 to the first secondary winding 106 at a same or close to same magnitude.
The set of cancellation secondary windings 108 and 109 may introduce a common mode cancellation waveform to cancel out the common mode noise waveform created by the switching device 101. As illustrated in FIG. 1, the set of cancellation secondary windings 108 and 109 may be placed on the primary side of the transformer 110. The set of cancellation secondary windings may be placed on the secondary side of the transformer 110, meaning the side of the transformer 102 that includes the regulating device 113. The set of cancellation secondary windings may include two or more cancellation secondary windings. For simplicity, the set of cancellation secondary windings are only illustrated on the primary side of the transformer 110.
As indicated by the placement of the dot on a right side of the set of cancellation secondary windings 108 and 109 in FIG. 1, the set of cancellation secondary windings 108 and 109 may be wound opposite in phase to the primary winding 104 and the first secondary winding 106. In other words, the set of cancellation secondary windings 108 and 109 are coiled in an opposite direction around the magnetic core 102 of the transformer 110 as compared to the primary winding 104 and the first secondary winding 106.
The set of cancellation secondary windings 108 and 109 may be coupled between the DC voltage output from the rectifier 118 and the switching device 101. The set of cancellation secondary windings 108 and 109 may be wired in a common mode configuration. One of the set of cancellation secondary windings 108 may be coupled in series between a DC voltage reference terminal 125 and one terminal of the switching device 101. Another of the set of cancellation secondary windings 109 may be coupled in series between another DC voltage reference terminal 126 and another terminal of the switching device 101.
The common mode cancellation waveform may be approximately equal in amplitude to the common mode noise waveform but the common mode cancellation waveform is opposite in phase, which creates the cancellation effect versus the common mode noise waveform. Under certain operating conditions, the magnitude of the common mode cancellation waveform may be equivalent to the magnitude of the common mode noise waveform. The magnitude of the common mode cancellation waveform may be equivalent because the number of turns of each of the set of cancellation secondary windings 108 may be equal to the number of turns of the primary winding 104 of the transformer 102. In other words, if the primary winding 104 has N turns, each of the set of cancellation secondary windings 108 has N turns. For example, the switching device 101 may generate a common mode noise waveform having a magnitude of 30 volts onto the primary winding 104 of the transformer 102. The primary winding 104 may have N, e.g., 4, turns. In order to cancel out the common mode noise waveform, each of the set of cancellation secondary windings 108 may have the same number of turns, e.g., 4 turns, which will produce a common mode cancellation waveform of 30 volts that is opposite in phase to the common mode noise waveform and cancels out the common mode noise waveform. The introduction of the common mode cancellation waveform may prevent the AC to DC power supply 100 from transmitting the common mode noise to the load 120.
While this noise cancellation circuit produces beneficial results, this noise cancellation circuit requires an additional capacitor and the set of cancellation secondary windings in order to operate.